The present invention relates to a transmission method and a transmission apparatus for transmitting signals on the basis of an OFDM-TDMA-system and further to a receiving method and a receiving apparatus for receiving signals transmitted by means of such a transmission method.
A transmission method and a transmission apparatus for transmitting signals on the basis of a OFDM/TDMA-system are explained relating to FIGS. 1-4 of the present application. In such a transmission method and apparatus, a plurality of subcarriers 1 being orthogonal to each other can be allocated to a variable number of channels U0, U1 . . . U9, each channel U0, U1 . . . U9 containing a variable number of subcarriers 1 depending on information to be transmitted as shown in FIGS. 1-4. FIG. 1 shows a group of ten frequency channels U0, U1 . . . U9. Each frequency channel U0, U1 . . . U9 can contain a variable number of subcarriers depending on information to be transmitted, as shown for the channels U0 and U1 in FIG. 2. The channel U0 contains a plurality of subcarriers 1, and the channel U1 contains a number of subcarriers 1 different from channel U0. In a transmission method and the transmission apparatus for transmitting signals on the basis of a OFDM/TDMA-system, a variable number of subcarriers 1 can be allocated to each channel depending on the amount of information to be transmitted. The channel U0 shown in FIG. 2 contains 21 subcarriers 1, whereas the channel U1 shown in FIG. 2 contains only 10 subcarriers 1. Therefore, the channel U0 can be transmitted by more than twice the transmission rate of the channel U0. On the border of each channel U0, U1 . . . U9, a single subcarrier having zero power is placed as guard band 2 to minimize interference to users placed in the adjacent frequency band or to fulfill certain spectrum masks. If the influence of an interference by the band in the neighborhood is small, the guard band 2 need not to be provided, whereas, when the influence is excessive, a plurality of guard bands 2 can be provided.
The subcarriers 1 are generated by orthogonal frequency division multiplex (OFDM) processing. As shown in FIG. 3, W(f) indicates a wave form indicating an energy on the frequency axis and B(Hz) indicates the distance between two adjacent subcarriers. The OFDM processing provides for a multi-subcarrier-system, wherein the number of channels which can be multiplexed is not limited by an interference from the other channels and can be freely determined depending on the bandwidth to be allocated. By changing the number of the subcarriers to be allocated to the different channels, it is possible to change the transmission rate or to achieve a variable transmission rate. The subcarriers between the respective channels can be easily separated by means of a filter, thereby making it possible to prevent deterioration of S/N characteristics. Since the OFDM processing is used for the multi-subcarrier modulation, a guard band S is not necessarily needed between different channels, thereby achieving a very high spectral efficiency. Further on, because fast Fourier transformation can be utilized, the necessary processing can be rapid and small.
Further on, the number of channels in each group of channels can be varied, as shown in FIG. 4. In FIG. 4, a group of six channels U0, U1 . . . U5 is shown. In a OFDM/TDMA-system, the number of channels in a group of channels can be varied within the system frequency band depending on information to be transferred.
In the known and standardized GSM-System, a type of single carrier frequency modulation called GMSK is used. The frequency channels are constant and the spacing (bandwidth) between adjacent frequency channels is 200 kHz. The number of FDMA-channels is 124 and a time division multiple access (TDMA) is used to support the number of parallel connections. The TDMA scheme in the GSM-System is 8 GSM-timeslots within one time frame. The GSM-timeslot length is 576,9 xcexcs (15/26 ms), as is shown in FIG. 5. As can be seen in FIG. 5, the transmitted GSM-timeslots are not fully occupied by the transmitted burst to reduce interference from adjacent GSM-timeslots if the system is not perfectly synchronized. The guard period is 8,25 bits, which corresponds to 30,5 xcexcs. The guard period is divided in two parts, wherein one of the parts is located at the beginning of the GSM time slot, and the other part is located at the end of the GSM-timeslot.
A GSM time frame consists of 8 GSM time slots and has therefore a length of 4615,4 xcexcs, as is shown in FIG. 6. The GSM-system supports slow frequency hopping, which is explained in FIG. 6. The shown GSM-timeslot 3 is a receiving timeslot. According to the time division duplex (TDD)-system of the GSM-system, a corresponding transmission GSM-timeslot 4 is transmitted some timeslots later. Further on, the GSM-system makes use of the frequency division duplex (FDD)-system with 45 MHz between uplink and downlink, so that the transmission GSM-timeslot 4 is transmitted in the corresponding uplink frequency band, when the receiving GSM-timeslot 3 had been sent in the uplink frequency band, or vice versa. The next succeeding receiving GSM-timeslot 5 is of course transmitted in the same uplink or downlink frequency band as the preceding GSM-timeslot 3, but in a different frequency channel, according to the slow frequency hopping. The frequency hopping improves, together with the interleaving procedure, the transmission of the signals in view of the frequency and interference diversity. The usual interleaving depth in the GSM-system is 36,923 ms corresponding to 8xc3x978 GSM-timeslots.
When transmitting signals between a base station and one or more mobile stations, the mobile channel introduces multipath distortion on the signaling wave forms. Both the amplitude and phase are corrupted as the channel characteristics changes because of movements of the mobile station. In order to perform a coherent detection of the transmitted signals, reliable channel estimates are required. This can be obtained by occasionally transmitting known data or so-called pilot symbols. The corresponding receiving side interpolates the channel information derived from the pilot symbols to obtain a channel estimate for equalizing the received data signal. The pilot symbol is thereby known both by the transmitting and the receiving apparatus.
The object of the present invention is therefore to provide a transmission method and a transmission apparatus for transmitting signals on the basis of an GSM compatible OFDM-TDMA-system and further a receiving method and a receiving apparatus for receiving such signals, which allow for a reliable channel estimation on the receiving side.
This object is achieved by a transmission method, a transmission apparatus, a receiving method and a receiving apparatus according to the claims.
Advantageous features of the present invention are defined in the respective subclaims.
The transmission method for transmitting signals on the basis of a OFDM/TDMA-system comprises the steps of
allocating a plurality of subcarriers being orthogonal to each other to a variable number of channels, each channel containing a variable number of subcarriers depending on information to be transmitted in said signals,
wherein, for the transmission of said signal in a GSM-system having a constant number of predetermined GSM-frequency channels and a constant number of predetermined GSM-timeslots being grouped in GSM-frames, a number of said subcarriers is allocated corresponding to the bandwidth of said GSM-frequency channels, so that a multiple of one resulting OFDM/TDMA-timeslots matches with one or a multiple of one GSM-timeslot,
wherein a pilot symbol is allocated to every n-th subcarrier, whereby n is an integer and n greater than 1, and transmitting said signals.
The transmission apparatus for transmitting signals on the basis of a OFDM/TDMA-system according to the present invention comprises
allocation means for allocating a plurality of subcarriers being orthogonal to each other to a variable number of channels, each channel containing a variable number of subcarriers depending on information to be transmitted in said signals, wherein
said allocation means, for the transmission of said signals in a GSM-system having a constant number of predetermined GSM-frequency channels and a constant number of predetermined GSM-timeslots being grouped in GSM-frames, allocate a number of said subcarriers corresponding to the bandwidth of said GSM-frequency channels so that a multiple of one resulting OFDM/TDMA-timeslot matches with one or multiple of one GSM-timeslot and for allocating a pilot symbol to every n-th subcarrier, whereby n is an integer and n greater than 1, and
transmission means for transmitting said signals.
In the presented transmission system, the signals are transmitted in or on the basis of an OFDM/TDMA-system, which is backward compatible to the standardized GSM-system. The transmission band of this OFDM/TDMA-system can be the same or can be different from the known GSM frequency band. A respective number of subcarriers of the OFDM/TDMA-system are allocated so that their bandwidth matches or corresponds to the bandwidth or a multiple of the bandwidth of the GSM frequency channels. Signals formed in an OFDM/TDMA system can in this way be transmitted and/or received also in an GSM-system.
The allocation of pilot symbols to every n-th subcarrier according to the present invention allows an accurate and reliable channel measurement and consequently a reliable correction of the received data signals on the receiving side. In case that a multiple of one OFDM/TDMA-timeslot matches with one GSM-timeslot, the pilot symbols in adjacent OFDM/TDMA-timeslots are advantageously frequency interlaced in respect to each other. Thereby, not only a frequency interpolation, but also a time interpolation of the channel transfer function on the receiving side is enabled to assure a reliable correction of the received data signals. The pilot symbols in adjacent OFDM/TDMA-timeslots can be symmetrically interlaced, whereby one pilot symbol is allocated to a subcarrier in the frequency middle between two respective pilot symbols of an adjacent OFDM/TDMA-timeslot. Thereby, an optimized interpolation on the basis of the received pilot symbols for estimating the channel transfer function on the receiving side can be performed.
In a further advantageous embodiment of the present invention, 48 of said subcarriers are allocated corresponding to the bandwidth of said GSM-frequency channels, so said 2 OFDM/TDMA-timeslots match with 1 GSM-timeslot, whereby n=6 or n=8. By choosing these parameters, the number of used pilot symbols is optimized. The use of pilot symbols for a channel function estimation introduces an overhead to the transmitted signals, which cannot be used for the transmission of data signals, and it is thus desirable to keep the number of pilot symbols low. On the other hand, a high number of pilot symbols is required on the receiving side to assure a reliable channel function estimate for a correction of the received data signals. The presented parameters define an optimized choice in view of these contradicting criteria. Further on, these parameters are chosen in view of the use of the present invention in an indoor environment, in which the channel transfer function, e.g. the channel attenuation, is a generally flat curve. In this case, only a small number of pilot symbols is necessary for a usable estimation. In an outdoor environment, however, a larger number of pilot symbols in each transmission channel, for example an GSM-frequency channel is necessary to enable the estimation of a usable channel transfer function. The reason is that in an outdoor environment, the channel transfer function, e.g. the channel attenuation, can have large variations due to multipath effects and a faster moving speed of the mobile stations. Therefore, it is necessary to tailor the pilot symbols to each base station side and also to choose the number of transmitted pilot symbols correspondingly. The parameters defined in the subclaims 4 and 8 are particularly useful in an indoor environment, in which the channel attenuation has a generally flat curve and the moving speed of the mobile stations is comparatively low.
In the transmission apparatus and the transmission method, the number of subcarriers to be allocated corresponding to the bandwidth of one GSM-frequency channel can be chosen, so that several OFDM/TDMA-timeslots are mapped into one GSM-timeslot, or several OFDM/TDMA-timeslots are mapped into several GSM-timeslots, e.g. eight GSM-timeslots (one GSM-frame). In the OFDM/TDMA-system, the transformation of one or a plurality of the subcarriers into the time domain results in a OFDM/TDMA-time burst. According to the present invention, one OFDM/TDMA timeslot contains essentially one OFDM/TDMA-time burst.
A very important consequence of the mapping of the OFDM/TDMA-timeslots into the GSM-timeslots is that the same interleaving depth as in a standard GSM-system can be obtained. A standard GSM-interleaving depth is 8xc3x978 GSM-timeslots (approx. 36,923 ms). In the present invention, one or more OFDM/TDMA-timeslots (e.g. two, four, . . . ) are mapped into one GSM-timeslot. Therefore, the information units to be transmitted according to the system of the present invention can be smaller than in the standard GSM-system. This is advantageous in view of the interleaving depth. If, for example, two OFDM/TDMA-timeslots are mapped into one GSM-timeslot, and 8 OFDM/TDMA-timeslots form one frame (8-TDMA), an interleaving depth of 8 frames (same as GSM) results in a total interleaving delay of 18,461 ms, which is half of the corresponding total interleaving delay of 36,923 ms in the GSM-system. Therefore, the transmission of information in a system according to the present invention can have a smaller overall delay with the same interleaving (frequency and interference diversity). An interleaving depth of 16 frames (approx. 36,923 ms) results in the same overall delay as in the standard GSM-system, but is much more reliable in view of transmission problems (time-, frequency- and interference diversity). For the transmission of speech signals, usually a smaller interleaving delay is desired due to the real time requirements. For example, for the transmission of speech signals interleaving depths smaller than 40 ms and short time-frames (4-10 ms) are advantageous. For the transmission of data signals, the real time requirements are not so important, so that a longer interleaving depth can be chosen to improve the data transmission reliability.
Advantageously, the signals to be transmitted are interleaved with a total interleaving delay corresponding to 8xc3x978 GSM-timeslots. Alternatively, the signals to be transmitted are interleaved with a total interleaving delay corresponding to 4xc3x978 GSM-timeslots.
Further on, the allocating step can comprise the steps of generating a clock, modulating a signal to be transmitted and producing said number of subcarriers according to said clock, transforming said subcarriers into time range bursts, and generating said OFDM/TDMA-timeslots by adding a guard time, a ramp time and an adaptation guard time to each of said time range bursts.
Correspondingly, the allocation means can comprise clock generation means for generating a clock, a modulation means for modulating a signal to be transmitted and producing said number of subcarriers according to said clock, a transformation means for transforming said subcarriers into time range bursts, and a timeslot generation means for generating said OFDM/TDMA-timeslots by adding a guard time, a ramp time and an adaptation guard time to each of said time range bursts.
According to further aspects advantageous, numbers of subcarriers to be allocated corresponding to the bandwidth of said GSM frequency channels are defined, so that the resulting OFDM/TDMA-timeslots match well into one or a multiple of one GSM-timeslot.
In the following description, the OFDM/TDMA signals are formed and transmitted in a GSM system. A number of OFDM/TDMA subcarriers is allocated to one or more GSM frequency channels in the standardized GSM transmission band. The present invention is, however, not limited to this example and the OFDM/TDMA transmission band can be different from the GSM transmission band. In this case, the OFDM/TDMA frequency channels are different from the GSM frequency channels. The subcarriers of the OFDM/TDMA system, however, are allocated so that their bandwidth matches or corresponds essentially to the bandwidth or a multiple of the bandwidth of the GSM frequency channels, to assure the compatibility.